In an image forming apparatus such as a copying machine or printer of the electrophotographic system, an electrostatic latent image formed on a photosensitive member is developed with a developer. The developer image formed on the photosensitive member is transferred to a transfer medium such as paper or OHP sheet as needed, and then fixed to the transfer medium by a method such as heating, pressing or use of solvent vapor.
As the developer, is used colored particles (toner) comprising a binder resin and a colorant. Developers are roughly divided into one-component developers with the surface of a toner coated with an external additive (flowability-imparting agent) such as silica and two-component developers composed of a toner and a carrier. The developers include magnetic developers making use of a magnetic toner comprising magnetic powder and non-magnetic developers making use of a toner comprising no magnetic powder. Non-magnetic one-component developers are recommended from the viewpoints of the miniaturization and weight saving of the image forming apparatus, and definition of images, etc.
There is yearly a strong demand for the improvement in the definition of images formed by the image forming apparatus of the electrophotographic system. As toners, toners (pulverized toners) obtained by a process (grinding process) comprising melting and mixing components such as a synthetic resin and a colorant and then grinding and classifying the resultant mixture to prepare colored particles have heretofore been used mainly. In recent years, attention has been attracted to toners (polymerized toners) composed of colored polymer particles obtained by a process in which a monomer composition comprising a polymerizable monomer and a colorant is subjected to suspension polymerization because they are easy to control their particle diameter, spherical and excellent in flowability and permit omitting a classification step.
Electrophotographic developers are required to have such characteristics or properties that they are excellent in flowability and shelf stability, do not give image defects such as occurrence of white stripes and attain almost constant image quality even when the ambient conditions including temperature and humidity fluctuate. However, it has been difficult to produce a toner and a developer satisfying these characteristics or properties, and various improved proposals have been made under the circumstances.
Japanese Patent Application Laid-Open No. 8-248676 has proposed an electrophotographic polymerized toner which is obtained by polymerizing a polymerizable monomer and has the relationship of 5 μS/cm≦D2−D1≦50 μS/cm, wherein D1 is an electrical conductivity of water, and D2 is an electrical conductivity of a filtrate obtained by dispersing the polymerized toner in the water in a proportion of 1 g per 20 ml of the water, fully stirring the dispersion to equilibrium and then filtering off the polymerized toner.
The publication shows that when this polymerized toner is mixed with a carrier composed of ferrite particles coated with a resin to use it as a two-component developer, a change in charge level is little even under different environments of low temperature and humidity and high temperature and humidity. In other words, this polymerized toner is charged by friction with magnetic iron powder.
When this polymerized toner is used as a non-magnetic one-component developer in an image forming apparatus of a system that the toner is charged by friction with a development roller or a development blade, however, the dependence of charge level on environment becomes high, and deterioration of image quality by environmental changes is observed. This toner is also insufficient in flowability and shelf stability.
Japanese Patent Application Laid-Open No. 11-72949 has proposed an electrophotographic developer comprising polymer particles (polymerized toner) for developer obtained by polymerizing a polymerizable monomer and an external additive attached to the surfaces of the particles, wherein the pH of a water extract obtained by a method (boiling extraction method) of dispersing the developer in ion-exchanged water having a pH of about 7 and boiling the dispersion is about 4 to 7.
This developer is greatly improved in deterioration of image quality by environmental changes and also improved in shelf stability and flowability. However, a further improvement is required to fully satisfy the requirement level for improvement in image quality.